I ran echo 'blog.myusername.me' >> source/CNAME in my Octopress repository, and then ran rake generate and rake deploy to deploy it to Github.

I went to Namecheap, and added a new CNAME under my myusername.me domain: "blog -> myusername.github.com -> CNAME".

Waited a couple hours.

However, now when I go to myusername.github.com, it redirects me to blog.myusername.me (which is my old Wordpress blog), which seems like the inverse of what I want. (Going to blog.myusername.me directly still shows my Wordpress blog.)

+1 Without more info about the situation I think this is the most likely cause. DNS changes take time to propagate. Here's some info on how to clear your local cache in Windows/OSX. docs.cpanel.net/twiki/bin/view/AllDocumentation/…. It may take additional time for the remote DNS servers to update to reflect the changes.
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Evan PlaiceApr 8 '12 at 6:01